Leek, cauliflower and cheddar soup

Ingredients

• 1 tablespoon oil
• 1 tablespoon butter
• 2 leeks, white and light green part only, finely sliced
• 1 clove garlic, chopped
• 1 medium-sized head cauliflower, cut into florets
• 2 floury potatoes, peeled and roughly chopped
• 5 cups chicken stock
• 5 slices prosciutto
• 150 grams vintage cheddar cheese, crumbled
• 2 tablespoons fresh chives, finely chopped
• salt and freshly ground black pepper